Reduced serum testosterone (T) is common among subfertile and infertile young men, including most men diagnosed with idiopathic infertility. Reduced T is also common in aging men, with T levels declining at age 40 and been low in the majority of men older than 60. Reduced T is often associated with mood changes, fatigue, depression, decreased lean body mass, reduced bone mineral density, increased visceral fat, metabolic syndrome, decreased libido and reduced sexual function. T replacement therapy (TRT) is used clinically to restore T levels. TRT can treat symptoms associated with low T. However, TRT may increase the risk and aggressiveness of prostate cancer, augment the incidence of adverse cardiovascular events, favor obesity and depression and even increase the rate of mortality in patients. Therefore is not recommended for patients at high risk of such diseases. Moreover, long-term TRT can suppress luteinizing hormone (LH) production, making this approach inappropriate for men who wish to have children. Fluctuating T levels, skin irritation, and T transfer to others through skin contact are additional disadvantages of TRT. The molecular mechanisms that govern androgen formation in testicular Leydig cells remain unclear. Identification of these mechanisms will facilitate development of new approaches for inducing endogenous T synthesis voiding exogenous T treatment.
T production is regulated by LH and its secondary messenger, cAMP. Cholesterol import from cytosolic sources into mitochondria is a hormone-sensitive and rate-limiting step of steroidogenesis. Cholesterol is cleaved into pregnenolone by CYP11A1 in mitochondria, and steroidogenesis begins. Cholesterol import into mitochondria is mediated by a hormone-induced multiprotein complex called the transduceosome, which is composed of cytosolic and outer mitochondrial membrane (OMM) proteins that control the rate of cholesterol entry into the OMM. These proteins include the cytosolic mitochondria-targeted, hormone-induced steroidogenic acute regulatory protein (STAR), the OMM high-affinity cholesterol-binding protein translocator protein (TSPO), which contains a cytosolic cholesterol recognition/interaction domain (CRAC) and the OMM voltage-dependent anion channel protein (VDAC1). Recent studies shed light on the importance of interactions between STAR, TSPO and VDAC1, suggesting that cholesterol import into mitochondria relies on the function and physical interactions between components of the transduceosome. The nature and dynamics of transduceosome protein-protein interactions remain unknown.
The 14-3-3 family of adaptor proteins were recently shown to have binding motifs on important functional sites in STAR, TSPO, and VDAC1 and 14-3-3γ was identified as a regulator of STAR activity. However this hormone-induced 14-3-3 isoform was shown to function in a transient manner at the initiation of steroidogenesis, to delay the maximum STAR activity. Indeed, the function of 14-3-3γ is terminated as it dissociates from STAR, allowing for maximal steroid production. In these studies, the levels of the 14-3-3 family ε isoform, were found to be increased in Leydig cell mitochondria during steroidogenesis. This isoform mediates in a tissue/target-specific manner, cell functions such as neural development, adipocyte differentiation, protein trafficking, cell cycle, apoptosis and cell signaling. Levels of 14-3-3ε, formerly known as mitochondrial import stimulating factor 25, are also high in human testes but its function in this tissue is unknown.
It would be desirable to be provided with a therapeutic agent capable of upregulating endogenous steroid production, such as testosterone production, without altering luteinizing hormone levels, to avoid or limit the side-effects listed above. It would also be desirable to be provided with screening assays for determining if a putative agent is capable of upregulating endogenous steroid production.